Coherent optical detection chains are known for detecting polarization-multiplexed signals after their propagation within an optical medium.
A coherent receiver is described in “Digital filters for coherent optical receivers” by Seb. J Savory (Optics Express, 21 Jan. 2008, Vol. 16, #2, pages 804 to 817). In this document, the receiver comprises an optical stage followed by an electronic stage. The optical stage receives the polarization-multiplexed signal after it has passed through an optical medium, often a birefringent one, such as an optical fiber. The optical stage particularly comprises an optical polarization splitter and a mixer for mixing polarization components of the received signal with the corresponding polarization components of a local oscillator signal; this optical stage of the receiver is sometimes called a 90° optical hybrid. Four analog electrical signals are obtained at the output of the optical stage and are delivered to the coherent receivers electronic stage. The coherent receiver is made up of multiple stages, the first one being an analog/digital conversion stage (A/D stage) for producing four digital electrical signals. These signals are then processed in multiple other electronic stages that may perform clock recovery, resequencing, chromatic dispersion compensation, polarization demultiplexing, carrier recovery, and symbol estimation functions. The coherent receiver delivers to electrical signals E1 and E2 which carry the data initially transported by the two polarization-multiplexed optical signals O1 and O2 injected into the optical medium.
One difficulty of coherent detection is knowing how to combine the two detected electrical signals E1 and E2 with the two optical signals injected into the optical connection, O1 and O2 without inverting them.
There is therefore a need to know how to identify the right match between the optical signals O1 or O2 and the detected signals E1 or E2.